The present invention generally relates to signal transmission method and apparatus, and more particularly to a signal transmission method and apparatus for an input unit of an information or data processing system in which the input unit having a number of contacts and a CPU is coupled to a main control circuit of the system through a serial transmission path.
The signal transmission system of the type mentioned above is disclosed in Japanese Unexamined Patent Application Publication No. 65349/1984. According to this hitherto known technique, the input unit includes a number of contacts switched (turned on or off) by selectively actuating a number of keys arrayed on a keyboard and a CPU for controlling data transmission and reception. The input unit repeatedly executes sequentially a key switch status extracting process for extracting the switched key, a data sending processing for sending the address of the extracted key to a main control circuit and a data reception processor for receiving signals from the main control circuit.
With a view to reducing the number of conductors for interconnecting the main control circuit incorporated in the system main body and the input unit, a half duplex serial transmission system is adopted in which the data sending and reception are carried out through a single signal line. In other words, a series of processes are cyclically and repetionally executed over an actuated switch extracting period T.sub.1, a data sending process period T.sub.2 and a data reception process period T.sub.3. According to the teachings disclosed in the literature cited above, the system can operate without involving collision among data with the single signal line. However, since the aforementioned process modes T.sub.1, T.sub.2 and T.sub.3 are repeated, the actual operated key switch extracting time is reduced to a value determined by ##EQU1##
In other words, when the key switch is operated in the time interval of (T.sub.2 +T.sub.3), the response is delayed until the time (T.sub.2 +T.sub.3) has elapsed. This means that when a given key switch is turned on and off within the period mentioned above, the turn-on operation of the key switch can not be extracted or detected, giving rise to a problem.